Recently, a reverse osmosis process or ultrafiltration process using semipermeable membranes made of cellulose acetate, polyamide or the like has become widely used in various fields such as production of fresh water from sea water, waste water treatment, control of electro-deposition bath, food industry and medical industry. What is important in these separation processes with membranes is the selective permeability of the membranes, i.e. solute rejection and flux. Particularly, it is not too much to say that the quantity of flux is a factor determining the economical value of the processes.
Up to date, a large number of studies have been made to increase the flux of semipermeable membranes used for reverse osmosis and ultrafiltration, and even the studies reported in patents and papers are too numerous to enumerate. Many of the studies are closely related to a membrane casting process, so that it may be said that the studies to increase the flux of semipermeable membranes are equivalent to a study of said process.
Of these studies, typical ones may be a study to produce asymmetric membranes having active surface skin layers of the smallest possible thickness, a study to produce an ultrathin membrane corresponding to the active surface skin layer followed by backing the membrane on a porous support into a composite, and the like. These studies may be a regular approach to the improvement of flux, but it is not always said that they provide a general and more practical method.
On the other hand, it is well known that there are acrylonitrile copolymers as a raw material for the production of semipermeable membranes. In spite of many trials to produce semipermeable membranes of acrylonitrile polymer, the membranes produced are low in solute rejection although they have a large flux. And, it is said that their solute rejection is particularly low when the solute is sodium chloride which is a low molecular electrolyte. This is the reason why semipermeable membranes of acrylonitrile polymer can not be used for reverse osmosis although they find a practical application in ultrafiltration.
Previously, in view of the good film forming property, thermal resistance, acid resistance and alkali resistance of acrylonitrile polymers, the inventors extensively studied a process for producing semipermeable membranes capable of rejecting even low molecular electrolytes such as sodium chloride at a high rate. As a result, the inventors succeeded in producing semipermeable membranes of acrylonitrile polymer having better thermal, mechanical and chemical properties and higher solute rejection than the conventional cellulose acetate membranes when used for reverse osmosis, by the epoch-making process of plasma-treatment of porous membranes of acrylonitrile polymer (Japanese Patent Publication No. 38988/1977). This process brought about a great advance that can be never attained by the prior arts to the field of semipermeable membranes of acrylonitrile polymer.